Conventional radiotherapy is based on CT and cone beam CT imaging for tumor and organ at risk delineation and for treatment guidance. Because of the superior soft tissue contrast of magnetic resonance imaging (MRI), MRI based radiotherapy is becoming increasingly popular. In order to achieve the best possible balance between tumor control and normal tissue complications, a radiation dose should be high in the tumor and as low as possible in the normal tissues. Furthermore, radiation dose delivery needs to be accurate and predictable, while at the same time maintaining sufficient MR image quality.